/Engine/Source/Runtime/Engine/Public/Elements/SMInstance/SMInstanceManager.h
There is no guarantee of which static mesh instance within the group will actually end up in the selection set, nor which static mesh instance within the group NotifySMInstanceSelectionChanged will be called for. It is the responsibility of the manager to handle this correctly by treating any static mesh instance within a selection group as one logical unit for all the static mesh instances within the group.
virtual void ForEachSMInstanceInSelectionGroup ( const [FSMInstanceId](API\Runtime\Engine\Elements\SMInstance\FSMInstanceId) & InstanceId, [TFunctionRef](API\Runtime\Core\GenericPlatform\TFunctionRef)< bool([FSMInstanceId](API\Runtime\Engine\Elements\SMInstance\FSMInstanceId))> Callback )